1. Field of the Invention
This invention relates to a twist-beam type rear suspension for a vehicle.
2. Description of the Prior Art
There has been known a twist-beam type rear suspension as disclosed in, for instance, Japanese Unexamined Utility Model Publication No. 58(1983)-90814. The twist-beam type rear suspension generally comprises left and right trailing arms which are pivoted on the vehicle body at the front ends for vertical swinging motion by way of rubber bushings, and a torsion beam connecting intermediate portions of the trailing arms, and left and right rear wheels are supported on the rear end of the respective trailing arms so that vertical displacement of the rear wheels relative to each other due to bumps and recesses on the road is limited by torsion counterforce of the torsion beam.
It is generally preferred in order to improve running performance of the vehicle that the toe of the rear wheels be controlled in addition to the turning of the front wheels in response to turning of the steering wheel. For example, when the steering wheel is turned by a slight angle while the vehicle is moving straight, it is preferred that the outer (with respect to the turning direction) rear wheel be caused to toe in to generate understeer tendency, thereby improving running stability of the vehicle. On the other hand, during hard cornering, it is preferred that the outer rear wheel be caused to toe out to weaken strong understeer tendency especially in the case of a front-engine front-wheel-drive type vehicle in which understeer tendency is stronger. In another aspect, it is generally preferred that when the steering wheel is turned while the vehicle is moving at a low speed, the outer rear wheel be caused to toe out to improve the heading performance of the vehicle. On the other hand, during hard cornering or lane changing at a high speed, it is preferred that the outer rear wheel be caused to toe in to ensure running stability.
Such behavior of the outer rear wheel can be obtained by controlling the toe of the rear wheel according to the side force generated by change of centripetal acceleration to act on the rear wheel when the steering wheel is turned. However, in the case of the conventional torsion-axle type rear suspension, when side force acts on the rear wheel, the rear wheel is merely caused to toe out. That is, when a side force generated by centripetal acceleration acts on the rear wheels, a force of the same direction as the side force acts on each of the rubber bushings on the front ends of the trailing arms, and at the same time a pair of forces act on the rubber bushings to rearwardly displace the bushing outwardly with respect to the turning direction and to forwardly displace the other bushing, thereby causing solely the outer rear wheel to toe out, since the pivotal axes of the trailing arms are on the front side of the line of action of the side force.
Further, in the rear suspension disclosed in Japanese Unexamined Patent Publication No. 58(1983)-20505, the trailing arms are mounted so that the pivotal axes of the trailing arms are inclined to intersect on the front side of the line joining the rubber bushings on the front ends of the trailing arms, thereby causing the compound elastic center of the bushings, i.e., the rotational instantaneous center, to approach the line of action of the side force acting on the rear wheels. In this rear suspension also, toe of the outer rear wheel cannot be controlled as desired though the degree of toe in or toe out can be limited.